The present invention relates to the splitting of extremely high power laser beams within or without a high power optical resonator.
Many applications involving the manipulation of laser beams require that the beam be split into two beams of comparable relative intensity. Beamsplitters are usually fabricated for this purpose by depositing a thin-film optical coating on a transmissive substrate (in fact, two coatings are often required: one on the front surface to provide a sufficiently high reflectance, and one on the back surface to eliminate reflections of the transmitted beam from that surface.) However, the optical beams from high power lasers are often capable of damaging or evaporating the coating, particularly if the beamsplitter is installed directly within the cavity of such lasers where the optical intensities and fluences can be many orders of magnitude greater than those outside of the laser cavity. Multi-mirror resonators, for example, incorporate such beamsplitters as an important and practical means of controlling the longitudinal mode structure and obtaining stable, single-frequency laser oscillation. But even in applications of high power laser beams outside of the laser resonator, such as optical autocorrelation, the beamsplitter must be able to withstand extremely high incident optical intensities and fluences, and provide roughly the same power into each of the resulting beams without damaging.
Apart from the ability to withstand optical damage, beamsplitters must often be designed to operate over a wide range of wavelengths. However, broadband coatings are difficult and expensive to design and manufacture, and often possess a reduced damage threshold. An example of an extremely high power and broadband laser system requiring an intracavity beamsplitter is the phase locked free-electron laser (FEL), in which the interferometric coupling of successive optical pulses induces temporal phase coherence among the otherwise randomly phased pulses in the resonator (see, for example, U.S. Pat. No. 5,130,994). In such a laser, the temporal and spectral features of the individual pulses are preserved, but the interpulse coherence of the output pulse train is characterized by an axial mode separation equal to the rf frequency of the linac instead of the round-trip frequency of the resonator. The most practical configurations of these lasers employ a multi-mirror resonator such as a Michelson or Fox-Smith interferometer, in which the beamsplitter provides the interpulse coupling. Such lasers have important applications in high resolution, nonlinear spectroscopy and, given the appropriate optics, can typically be tuned over several octaves.
In extracavity applications of beamsplitters, such as optical autocorrelation, the power requirements of a given coating may be relaxed somewhat by increasing the area of the incident beam, a solution which is usually not available for intracavity applications. However, broadband designs remain an important requirement for applications to FELs and other tunable sources such as dye lasers. For example, optical autocorrelators are used to measure the duration of ultrashort optical pulses from each of these lasers, and are usually limited in bandwidth and tunability by the beamsplitter. These devices operate by splitting an incident pulse into two pulses of roughly equal intensity and then recombining them with a variable degree of temporal delay; the pulse duration is then inferred from the resulting interference signal. In such applications, thin-film coatings can entail an additional complication: improperly designed coatings can result in a temporal distortion of the optical pulse as it propagates through and is reflected from the beamsplitter.